New Upper Build Locking Up After Firing

[montana]

Are there any marks on the barrel extension ?? If the barrel is cocked slightly off, there will be marks on the sides of the barrel extension lugs.. A barrel that is slightly canted can cause the un-locking problems you are talking about. The easiest way to test for this is to insert a Midwest Industries action rod in your upper receiver. If the barrel is canted at all, the action rod lugs will not seat into the barrel extension..https://www.midwestindustriesinc.com/MI-...mi-urr.htm